As the use of the Internet spreads, the amount of data communicated is rapidly increasing, making the optical fiber communication very important. In the optical fiber communication, electric signals are converted into optical signals, and the optical signals are transmitted through optical fibers. The optical fiber communication is characterized in that the signals are transmitted in the broad band, with a small loss, and are not affected by noise.
Known as systems for converting electric signals into optical signals are the direct modulation system using a semiconductor laser and the external modulation system using optical modulators. The direct modulation system need not use the optical modulators and its running cost is low, but cannot achieve high-speed modulation. This is why the external modulation system is used in high-speed and long-distance data communication.
Optical modulators in which an optical waveguide is formed by Ti (titanium) diffusion in the vicinity of a surface of a lithium niobate single crystal substrate have been practically used. Although high-speed optical modulators having a modulation speed of 40 Gb/s or more are commercially available, they have a major drawback that the entire length thereof is as long as about 10 cm.
On the other hand, Patent Documents 1 to 4 disclose a Mach-Zehnder optical modulator using a ridge type optical waveguide obtained by processing a c-axis oriented lithium niobate film formed by epitaxial growth on a sapphire single crystal substrate into a ridge type. The optical modulator using the lithium niobate film achieves a significant reduction in size and driving voltage as compared with the optical modulator using the lithium niobate single crystal substrate.